ESD protection circuit with isolated SCR for negative voltage operation

ABSTRACT

A semiconductor controlled rectifier (FIG.  4 A) for an integrated circuit is disclosed. The semiconductor controlled rectifier comprises a first lightly doped region ( 100 ) having a first conductivity type (N) and a first heavily doped region ( 108 ) having a second conductivity type (P) formed within the first lightly doped region. A second lightly doped region ( 104 ) having the second conductivity type is formed proximate the first lightly doped region. A second heavily doped region ( 114 ) having the first conductivity type is formed within the second lightly doped region. A buried layer ( 101 ) having the first conductivity type is formed below the second lightly doped region and electrically connected to the first lightly doped region. A third lightly doped region ( 102 ) having the second conductivity type is formed between the second lightly doped region and the third heavily doped region. A fourth lightly doped region ( 400 ) having the second conductivity type is formed between the second lightly doped region and the third heavily doped region and electrically connected to the second and third lightly doped regions.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of and claims priority to U.S. patentapplication Ser. No. 13/668,022 filed Nov. 2, 2012. Said application isincorporated herein by reference.

BACKGROUND OF THE INVENTION

Embodiments of the present embodiments relate to an isolatedsemiconductor controlled rectifier (SCR) circuit for electrostaticdischarge (ESD) protection. A preferred embodiment of the circuit isintended for use at input, output, or input-output terminals having anegative operating voltage with respect to GND or VSS, but the circuitmay also be used between power supply terminals such as VDD and GND orVSS terminals of an integrated circuit.

Referring to FIGS. 1-3, there is an ESD protection circuit of the priorart that is similar to those disclosed by Ker et al. in U.S. Pat. No.6,765,771. The plan view of FIG. 1 illustrates a dual semiconductorcontrolled rectifier formed on p-type substrate (PSUB) 102 andsurrounded by n-type well (NWELL) 100. The PSUB layer 102 iselectrically connected to P+ region 112. The dual SCRs are formedsymmetrically above and below P+ region 112, so only the lower SCR willbe described in detail. Here and in the following discussion the samereference numerals are used to identify the same or similar circuitelements in the various drawing figures. N+ region 114 is the cathode ofthe lower SCR and is formed adjacent P+ region 112 within p-type wellregion 104. P+ region 108 is formed within NWELL 100 and serves as ananode for the lower SCR. N+ region 106 is electrically connected toNWELL 100. Gate region 110 is formed over a boundary between NWELL 100and PSUB 102. Gate region 110, P+ anode 108, and N+ region 106 areelectrically connected to reference terminal 122, which is preferablyGND or VSS. P+ region 112 and N+ region 114 are electrically connectedto terminal 120, which is preferably an input, output, or input-outputterminal to be protected.

Referring next to FIG. 2, there is a cross sectional diagram of thelower SCR along the plane A-A′ as indicated by the line in FIG. 1. TheSCR is formed on P-type substrate (PSUB) 200. N-type buried layer (NBL)101 is formed in PSUB 200 below the surface by ion implantation. Takentogether n-type well (NWELL) 100 and NBL 101 form an isolated P-typeregion (PSUB) 102. The lower SCR includes P+ anode 108 formed in NWELL100 and N+ cathode 114 formed in p-type well region 104. Active P+regions 112 and 108, N+ regions 114 and 106, and the channel regionbelow gate 110 are separated by shallow trench isolation (STI) regions124.

Turning now to FIG. 3, there is a simplified diagram of the SCR of FIG.2 showing individual bipolar transistors. Shallow trench isolation (STI)regions are omitted for clarity. The lower SCR of FIG. 2 comprises avertical SCR and a horizontal SCR. The vertical SCR includes PNPtransistor 304 and NPN transistor 306 and forms a vertical current pathfrom P+ anode 108 to NBL 101 via NWELL 100 and back to N+ cathode 114via PSUB 102. The horizontal SCR includes PNP transistor 300 and NPNtransistor 302 and forms a horizontal current path directly from P+anode 108 to N+ cathode 114. Parasitic resistor 301 is a base-emittershunt resistor for PNP transistor 300. Parasitic resistor 303 is abase-emitter shunt resistor for NPN transistors 302 and 306.

Several problems arise with operation of the SCR of FIGS. 1-3 that limitoperating voltage, gain of the SCR, and introduce reliability problemsas will become apparent in the following discussions. Variousembodiments of the present invention are directed to solving theseproblems and improving operation of the SCR without increasing processcomplexity.

BRIEF SUMMARY OF THE INVENTION

In a preferred embodiment of the present invention, a semiconductorcontrolled rectifier for protecting an integrated circuit is disclosed.The semiconductor controlled rectifier includes a first lightly dopedregion having a first conductivity type formed proximate a secondlightly doped region having a second conductivity type. A first heavilydoped region having the second conductivity type is formed within thefirst lightly doped region. A second heavily doped region having thefirst conductivity type is formed within the second lightly dopedregion. A buried layer having the first conductivity type is formedbelow the second lightly doped region and electrically connected to thefirst lightly doped region. A third lightly doped region having thesecond conductivity type is formed between the second lightly dopedregion and the third heavily doped region. A fourth lightly doped regionhaving the second conductivity type is formed between the second lightlydoped region and the third heavily doped region and electricallyconnected to the second and third lightly doped regions.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a plan view of an enclosed semiconductor controlled rectifier(SCR) of an electrostatic discharge (ESD) protection circuit of theprior art;

FIG. 2 is a cross sectional diagram along the plane A-A′ of the SCR ofFIG. 1;

FIG. 3 is a schematic diagram showing individual transistors of the SCRof FIG. 2;

FIG. 4A is a cross sectional diagram of a first embodiment of an SCR ofthe present invention;

FIG. 4B is a current-voltage diagram of the SCR of FIG. 4A;

FIG. 5A is a cross sectional diagram of a second embodiment of the SCRof the present invention;

FIG. 5B is a doping profile of p-type well 104 of FIG. 5A;

FIGS. 6A-6C are cross sectional diagrams of a third embodiment of theSCR of the present invention;

FIG. 7 is a cross sectional diagram of a fourth embodiment of the SCR ofthe present invention; and

FIG. 8 is a cross sectional diagram of a fifth embodiment of the SCR ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

The preferred embodiments of the present invention provide significantadvantages over electrostatic discharge (ESD) protection circuits of theprior art as will become evident from the following detaileddescription.

Referring to FIG. 4A, there is a cross sectional diagram of a firstembodiment of a semiconductor controlled rectifier (SCR) of the presentinvention. Here and in the following discussion, SCR refers to asemiconductor controlled rectifier rather than a silicon controlledrectifier, which is a special case of a semiconductor controlledrectifier. In general, heavily doped means a semiconductor region havinga concentration of 1e18 Acm⁻³ or greater. Likewise, lightly doped meansa semiconductor region having a concentration of less than 1e18 Acm⁻³.In both cases, the doped regions may be formed by ion implantation orother methods as are well known to those having ordinary skill in theart. Furthermore, the drawing figures are not to scale but are drawn toclearly illustrate important features of the present invention. In thefollowing discussion, the term “electrically connected” means an ohmiccurrent path exists between two or more cited elements and does notpreclude the existence of resistors, parasitic elements, or othercircuit elements within the current path as is well known in the art.

The SCR of FIG. 4A is formed on P-type substrate (PSUB) 200. N-typeburied layer (NBL) 101 is formed in PSUB 200 below the surfacepreferably by ion implantation. Taken together n-type well (NWELL) 100and NBL 101 form an isolated p-type region (PSUB) 102 with the sameimpurity type and concentration as PSUB 200. The SCR includes P+ anode108 formed in NWELL 100 and N+ cathode 114 formed in p-type well region104. N+ region 106 electrically connects NWELL region 100 to referenceterminal 122, which may be VSS, ground, or another suitable referenceterminal. Reference terminal 122 is also connected to P+ anode 108 andgate region 110. P+ region 112 electrically connects p-type well 104 toterminal 120, which may be an input, output, input-output, or anotherreference terminal. Terminal 120 is also connected to N+ cathode 114.Active P+ regions 112 and 108, N+ regions 114 and 106, and the channelregion below gate 110 are separated by shallow trench isolation (STI)regions 402 and 404. STI region 404 may optionally be omitted to improvethe gain of the horizontal SCR.

The present inventors have discovered one of the problems with the SCRof FIGS. 1-3 is the limited operating voltage at terminal 120 withrespect to reference terminal 122. Present analog circuits may requirean operating voltage of −10 V or more at terminal 120 with respect toreference terminal 122. For small feature sizes, however, it wasdiscovered that −6 V applied to terminal 120 with respect to referenceterminal 122 would fully deplete lightly doped PSUB region 102 and causecollector-emitter punch through of NPN transistor 306. This problem issolved by a first embodiment of the present invention in which p-typeregion (PBL) 400 is preferably formed by ion implantation between p-typeregion 104 and NBL 101. PBL 400 is preferably formed with an impurityconcentration between that of p-type region 104 and PSUB 102. If the PBLconcentration is too low, collector-emitter punch through of NPNtransistor 306 may still occur within the operating voltage range ofterminal 120. Alternatively, if the concentration of PBL 400 is toohigh, lateral collector-base avalanche conduction of NPN transistor 302will occur.

Referring now to FIG. 4B, there is a current-voltage diagram of the SCRof FIG. 4A. The absolute voltage at terminal 120 with respect toterminal 122 is shown along the horizontal axis. The absolute currentbetween terminal 120 and 122 is shown along the vertical axis. It isimportant to note that the trigger voltage of the SCR is now 16 V,rather than 6 V, with no evidence of collector-emitter punch through.Moreover, the holding voltage of the SCR is less than 1 V to provideeffective protection against electrostatic discharge (ESD).

Turning now to FIG. 5A, there is a second embodiment of the presentinvention. Here, however, p-type well layer 104 is modified accordingthe impurity concentration diagram of FIG. 5B. The original dopingprofile is produced by a Boron implant dose of 4e12 Acm⁻² and energy of15 keV. In addition, a second Boron implant dose of 1.8e12 Acm⁻² andenergy of 47 keV and a third Boron implant dose of 2e12 Acm⁻² and energyof 115 keV are performed. The three implants produce three respectiveoverlapping Gaussian distributions of FIG. 5A. The first and originalimplant produces a maximum concentration of 1.8e16 Acm−3 at 0.5 μm. Thesecond implant produces a maximum concentration of 1.5e16 Acm−3 at 1.6μm. The third implant produces a maximum concentration of 1.0e16 Acm−3at 3.4 μm. The resulting doping profile of FIG. 5A advantageouslyprevents collector-emitter punch through of NPN transistor 306 to −20 Vat terminal 120 with respect to reference terminal 122. Theprogressively decreasing concentration of Boron with increasing implantdepth advantageously prevents collector-base avalanche conduction of NPNtransistor 306. Since the maximum Boron concentration of the second andthird implants is no greater than the original implant, no lateralcollector-base avalanche of NPN transistor 302 will occur. Finally, noadditional masks are required for the second and third Boron implants.

Referring now to FIG. 6A, there is a first part of a third embodiment ofthe present invention. The operating voltage of the SCR of FIGS. 1-3 islimited by the electric field between gate 110 and p-type well region104. This limitation becomes increasingly significant as the dielectricthickness under gate 110 decreases. In the embodiment of FIG. 6A, gate110 may be necessary when a metal silicide layer is formed over activeregions 106, 108, 112, 114, and gate 110. The gate 110 prevents metalsilicide formation that might otherwise short p-type well region 104 toNWELL 100. In the embodiment of FIG. 6A, a lightly doped p-type wellregion 600 is formed by ion implantation between p-type well region 104and NWELL 100. P-type well region 600 is electrically connected top-type well region 104 and extends NWELL 100. Region 600 is preferablymore lightly doped than either of p-type well region 104 or NWELL 100 toavoid avalanche conduction with NWELL 100. During normal circuitoperation, when terminal 120 is negative with respect to terminal 122,an inversion layer connected to NWELL 100 forms at the surface of p-typewell region 600. This advantageously avoids any high electric fieldacross the dielectric layer under gate 110 during normal circuitoperation as well as during ESD events.

FIG. 6B is a second part of the third embodiment of the presentinvention. Here, when the space between SCR anode 108 and cathode 114 issufficiently large it is possible to omit implant region 600 so thatregion 602 is an extension of PSUB 102. Isolation region 604 may also beoptionally omitted to improve the gain of the horizontal SCR. As withFIG. 6A, an inversion layer connected to NWELL 100 forms at the surfaceof p-type region 602. This advantageously avoids any high electric fieldacross the dielectric layer under gate 110 during normal circuitoperation as well as during ESD events.

FIG. 6C is a third part of the third embodiment of the presentinvention. In the embodiment of FIG. 6C, a lightly doped n-type wellregion 606 is formed by ion implantation between p-type well region 104and NWELL 100. N-type well region 606 is electrically connected to NWELL100 and extends to p-type well region 104. Region 606 is preferably morelightly doped than either of p-type well region 104 or NWELL 100 toavoid avalanche conduction with p-type well region 104. During normalcircuit operation, when terminal 120 is negative with respect toterminal 122, region 606 under gate 110 remains in accumulation andconnected to NWELL 100. This advantageously avoids any high electricfield across the dielectric layer under gate 110 during normal circuitoperation as well as during ESD events. Alternatively, when not limitedby punch through and avalanche conduction, NWELL 100 may be extended tothe left to underlie gate 110.

Referring next to FIG. 7, there is a fourth embodiment of the presentinvention. Here, gate 110 and STI 404 (FIG. 6) are removed. Silicideblocking layer 710 is formed over portions of p-type well region 104 andNWELL 100 between N+ cathode 114 and P+ anode 108. The silicide blockinglayer is preferably a thin deposited dielectric layer such as SiO₂ or acomposite dielectric of SiO₂ and SiN. The silicide blocking layer 710advantageously prevents formation of metal silicide that would otherwiseshort p-type well 104 to NWELL 100. Moreover, gate 110 is removed sothere is no high electric field across the dielectric layer 710 duringnormal circuit operation as well as during ESD events. Removal of STIregion 404 advantageously improves the gain of the lateral SCR (FIG. 3)by creating a more direct collector-emitter current path for NPNtransistor 302.

Referring now to FIG. 8, there is a fifth embodiment of the presentinvention. Recall from the discussion of FIG. 3 that resistor 301 is abase-emitter shunt resistor for lateral PNP transistor 300. The presentinventors have determined that the gain, trigger voltage, and holdingvoltage of the lateral SCR formed by PNP transistor 300 and NPNtransistor 302 are significantly affected by the value of shunt resistor301. In order to improve these characteristics of the SCR, p-type(PWELL) region 800 is formed by ion implantation below and electricallyconnected to P+ anode 108. PWELL region 800 has a slightly higherimpurity concentration than NWELL 100 and preferably extends to NBL 101.However, one of ordinary skill in the art having access to the instantspecification will understand that SCR characteristics are improved evenwhen PWELL 800 extends any distance below P+ anode 108. This improvementis due to an increase in the value of resistor 301, since the currentpath through resistor 301 must flow through NWELL 100, below PWELL 800,and up to N+ well contact 106. Therefore, the maximum increase in thevalue of resistor 301 is advantageously achieved when PWELL 800 extendsto NBL 101.

Still further, while numerous examples have thus been provided, oneskilled in the art should recognize that various modifications,substitutions, or alterations may be made to the described embodimentswhile still falling within the inventive scope as defined by thefollowing claims. For example, although the foregoing discussion isspecifically directed to an SCR having a negative operating voltage atterminal 120 with respect to reference terminal 122, embodiments of thepresent invention are equally applicable to an SCR having a positiveoperating voltage at an input, input-output, or output terminal such asterminal 122 with respect to a reference terminal such as terminal 120.Moreover, although five embodiments of the present invention have beendiscussed separately, it is to be understood that many of them may becombined in a single improved SCR. Other combinations will be readilyapparent to one of ordinary skill in the art having access to theinstant specification.

What is claimed is:
 1. A semiconductor controlled rectifier, comprising:a first lightly doped region having a first conductivity type; a firstheavily doped region having a second conductivity type formed within thefirst lightly doped region; a second lightly doped region having thesecond conductivity type formed at a first time proximate the firstlightly doped region; a second heavily doped region having the firstconductivity type formed within the second lightly doped region; aburied layer having the first conductivity type formed below the secondlightly doped region and electrically connected to the first lightlydoped region, wherein the first lightly doped region extends to theburied layer; and a third lightly doped region formed at a second timebetween and touching both the first lightly doped region and the secondlightly doped region, wherein the third lightly doped region isshallower than the first lightly doped region and deeper than the secondlightly doped region and wherein the third lightly doped region has thesecond conductivity type and is electrically connected to the secondlightly doped region.
 2. A semiconductor controlled rectifier as inclaim 1, wherein a shallow trench isolation region is omitted betweenthe first heavily doped region and the second heavily doped region.
 3. Asemiconductor controlled rectifier as in claim 1, comprising a fourthlightly doped region having the second conductivity type formed betweenthe first heavily doped region and the buried layer and electricallyconnected to the first heavily doped region.
 4. A semiconductorcontrolled rectifier as in claim 1, comprising a gate formed over ajunction between the first lightly doped region and the second lightlydoped region, wherein the gate is electrically connected to the firstheavily doped region.
 5. A semiconductor controlled rectifier as inclaim 1, comprising: a fourth heavily doped region having the firstconductivity type and electrically connected to the first heavily dopedregion and the first lightly doped region; and a fifth heavily dopedregion having the second conductivity type and electrically connected tothe second heavily doped region and the second lightly doped region. 6.A semiconductor controlled rectifier as in claim 1, wherein thesemiconductor controlled rectifier is formed on a substrate having thesecond conductivity type, and wherein the second lightly doped region iselectrically isolated from the substrate by the first lightly dopedregion and the buried layer.
 7. An integrated circuit, comprising: asubstrate of a second conductivity type; and semiconductor controlledrectifier including: a first lightly doped region having a firstconductivity type; a first heavily doped region having the secondconductivity type formed within the first lightly doped region; a secondlightly doped region having the second conductivity type formedproximate the first lightly doped region; a second heavily doped regionhaving the first conductivity type formed within the second lightlydoped region; a buried layer having the first conductivity type formedin the substrate below the second lightly doped region and electricallyconnected to the first lightly doped region, wherein the first lightlydoped region extends to the buried layer; and a third lightly dopedregion formed between and touching both the first lightly doped regionand the second lightly doped region, wherein the third lightly dopedregion is shallower than the first lightly doped region and deeper thanthe second lightly doped region and wherein the third lightly dopedregion has the second conductivity type and is electrically connected tothe second lightly doped region.
 8. An integrated circuit as in claim 7,wherein a shallow trench isolation region is omitted between the firstheavily doped region and the second heavily doped region.
 9. Anintegrated circuit as in claim 7, comprising a fourth lightly dopedregion having the second conductivity type formed between the firstheavily doped region and the buried layer and electrically connected tothe first heavily doped region.
 10. An integrated circuit as in claim 7,comprising a gate formed over a junction between the first lightly dopedregion and the second lightly doped region, wherein the gate iselectrically connected to the first heavily doped region.
 11. Anintegrated circuit as in claim 7, comprising: a fourth heavily dopedregion having the first conductivity type and electrically connected tothe first heavily doped region and the first lightly doped region; and afifth heavily doped region having the second conductivity type andelectrically connected to the second heavily doped region and the secondlightly doped region.
 12. An integrated circuit as in claim 7, whereinthe second lightly doped region is electrically isolated from thesubstrate by the first lightly doped region and the buried layer.